The objective of this study is to characterize specific neurotransmitter/receptor dysfunction in 5-HT pathways in depressed patients and the time course of antidepressant-induced changes. The study will use PET imaging of serotonin receptors to characterize the changes in 5-HT function across different brain regions in depressed patients. A longitudinal design will be used to obtain in vivo quantitation of cortical and subcortical 5-HT2 receptors at three points during antidepressant treatment: prior to treatment, 2 days after treatment, and 8 weeks after treatment.